1. Technical Field
The present invention relates to locating a container in a stowage location on a container ship during loading and unloading of the container ship in a container terminal to enable tracking containers when they are loaded or unloaded at the container terminal.
2. Related Art
Over the recent decade, the number of shipping containers handled in container terminals has increased dramatically, which demands the container terminal operators increase their container handling efficiency. In particular, the efficiency of loading and unloading container ships is of great interest due to its direct effects on the ship turnaround.
For loading and unloading container ships, container cranes (also known as container handling gantry cranes or ship-to-shore cranes) are typically used. During loading and unloading of container ships, the container crane operator must be informed about the containers to be loaded or unloaded. While the stowage plan of a container ship is transmitted electronically from the departure port to the destination port, the crane operator may receive work instructions electronically (e.g., on a display) during the operations or in paper form before commencing the loading or unloading operations. The work instructions specify the stowage location of containers to be loaded into or unloaded from the container ship. The crane operator sits at the head of the crane, moves the head to the specified stowage location, and picks up a container from or drops off a container to the specified stowage location.
Two major issues with the above typical practice greatly limit the efficiency of container handling efficiency. First, although ideally the crane operator should be able to move the crane head to the stowage location specified in the work instruction to pick up or drop off a container, the crane operator from time to time makes wrong judgments about where the crane head is relative to the container ship and conducts container operations at wrong stowage locations. Therefore, clerks are often assigned to each crane to (1) manually check the ID of the container unloaded from the ship to ensure its correctness and (2) verify the location of a container as it is loaded into the ship. Such practice not only requires more staffing at a container terminal but also increases the handling time for each container. Some container terminals install Optical Character Recognition (OCR) systems on cranes to help verify the ID of the containers being unloaded from the ship, thereby reducing the requirements on staffing; however, OCR systems sometimes have difficulty in recognizing container IDs especially when the IDs are obscured for various reasons. Moreover, OCR systems cannot verify whether a container is loaded to the intended stowage location during the loading of a container ship.
Second, to increase container handling efficiency, inventory tracking systems are commonly used in container terminals. However, such systems are limited to the automatic tracking of containers inside a container terminal; clerks still need to manually create an entry in the inventory tracking system for every container that has been unloaded from a container ship and manually remove an entry or mark the entry as departed once the corresponding container has been loaded into a container ship. The primary reason is that those inventory tracking systems typically employ real-time positioning technology (such as Global Positioning System (GPS) and Real-time Locating System (RTLS)) and wireless communications to track locations of containers by actively tracking the movement and locations of container handling equipment (CHEs) that pick up, move, and set down the containers. This alone is not adequate for tracking containers for loading and unloading container ships: even if a container's position is know (e.g., based on the position of the crane's load-carrying member or spreader for handling the container), its corresponding stowage location in a container ship is unknown because (1) the container ship's position (including the orientation of the ship) is unknown and (2) the container ship's position shifts dynamically due to various factors such as tide and loading conditions. For example, a container ship can go up and down 6 to 8 feet twice a day due to tide; a one-degree orientation shift can create a position shift over 5 meter (more than twice the width of a typical container) for a container ship with length of 300 meters.